1. Field of the Invention
The invention relates to a propulsive apparatus for displacing a main fluid within a duct, and resembles a jet pump or ejector pump as used for pumping fluids, or solids suspended in fluids.
2. Prior Art
Jet pumps or ejector pumps have been used for many years and are characterised by a duct into which one or more jets of driving liquid are directed. The driving liquid propels the main fluid within the duct by transfer of the momentum of the driving liquid to the main fluid. The present invention relates to a particular type of jet pump known as a "peripheral jet pump", in which a series of jets are spaced around the periphery, thus leaving the centre of the duct free of obstructions. This permits the pump to pump objects suspended in the main fluid having a maximum size which is slightly less than cross-section of the duct. Many applications for this type of pump have been devised, such as pumping fish, fruit, or vegetables or other objects that are fragile and would be damaged by other types of pumps or conveying systems.
In general, prior art jet pumps are characterised by relatively low efficiency when compared with conventional fluid pumps, but this low efficiency is tolerated for the benefits of relatively gentle handling of delicate solids. The low efficiency of prior art jet pumps requires the use of relatively high powered units to pump the driving liquid, and this results in high energy costs and considerable space requirements where the pump is to be used. When such pumps are used for pumping fish on small fishing boats, the size of the power plant necessary to power the driving liquid can become excessive and thus limits the use of such pumps. An example of an ejector pump using two sets of jet nozzles inclined at different angles to the duct axis and spaced along the axis is shown in U.S. Pat. No. 4,155,682, issued to Hillis. Spacing between the two sets of jet nozzles increases length of the jet pump necessary for mixing of the fluids, and use of two separate sets of nozzles increases cost of the pump.
The height to which the pump can maintain a column of water is limited by pump efficiency and available power. When using various types of pumps, it is known to inject air into the column of water and fish as it is drawn from a fish net or hold of a boat. The injected air reduces effective density of the liquid or liquid/solid mixture within the column and this is commonly called an "air assist" lift pump. An example of a fish pump using air assist is shown in U.S. Pat. No. 2,736,121, issued to Kimmerle. This pump is characterised by complexity, and therefore cost and the number of parts which can damage fish drawn through the pump. A pump using air assist in combination with a jet pump is shown in Japanese publication No. 56-41500, but this has a central obstruction in the duct which limits size of product being handled and also could damage the product.